How do I get “rainbow toning” on a coin?

Todd V. asks: How do I get a rainbow toning on a coin? Or what is the chemical process to make a rainbow tone?

“Rainbow” toning or patina is an optical effect caused by graduated layers of silver oxide on the surface of a coin. This is often seen on coins that have been in contact with a sulfur source (a great oxidizer) for a good number of years (usually 20 years or more). These coins, if also in high grade uncirculated condition, are highly prized by coin connoisseurs for their extreme beauty.

Usually the sulfur source is not uniformly in contact with the coin. For example, old time coin albums were manufactured with large amounts of sulfur. The edges of the coin were in direct contact with the cardboard that suspended the coin in its place in the album. Oxidation emanates from the edge where it is thicker, toward the center which is further away from the sulfur source. The thicker the oxide coating the darker the toning. The graduations of oxide create light refraction that show the spectrum from black and dark blue (thick oxide) to reds and golds (thin oxide). The same effect was seen on bags of Morgan and Peace dollars that were stored for many years in government vaults. The canvas bags were strong sources of sulfur compounds and coins in contact with bag also exhibit similar rainbow toning seen in the albums.

The main difference between artificial and natural toning is how the oxide and therefore colors are distributed throughout the coin’s surface. Artificial toning usually shows abrupt changes in color versus the continuous graduations often seen on naturally toned coins. Also, artificially toned coins are often first chemically stripped of all toning then treated with sulfur compounds or other chemicals and then heated. When one chemically removes the oxide coating, the coin’s flow lines that are present on a uncirculated coin are partially removed. This results in a dull coin or one with a significant loss of luster. Though coin “improvers” have become adept at simulating natural oxide toning, careful observation can often identify these creations.